Existing Nd:YAG-pumped dye lasers or optical parametric oscillators may be used for combustion diagnostic techniques such as absorption spectroscopy, cavity ring-down spectroscopy (CRDS), laser-induced fluorescence (LIF), laser-induced polarization spectroscopy (LIPS), and degenerate four-wave mixing (DFWM). These existing sources typically have frequency bandwidths of the order of 3 GHz and exhibit multiple longitudinal frequency modes. The use of these multi-longitudinal-mode laser sources can cause numerous complications in signal analysis and greatly increase pulse-to-pulse signal fluctuations in techniques such as LIPS and CRDS.
Injection-seeded LiNbO3 optical parametric oscillators (OPO) with a resonant cavity locked to the frequency of the seed beam have been developed. However, the injection-seeding laser source is a continuous wave (cw) Nd:YAG laser and is not tunable. An OPO with beta-barium-borate (β-BBO) crystals has also been developed. This OPO system was pumped with the 355-nm third-harmonic beam from a single-mode Nd:YAG laser, and injection seeding with the pulsed 532-nm and 1064-nm beams from the same laser was also demonstrated. One attempt has seeded an OPO with a tunable cw color center laser at 1500 nm. The cavity was locked to the frequency of the seed laser beam. Other attempts have used a cw titanium-doped sapphire laser to seed an actively-locked ring cavity OPO.
Other related developments include injection-seeded systems with cavities resonant at the seed wavelength, and cavities that incorporate periodically poled lithium niobate (PPLN) crystals. A high-resolution OPO that is seeded with a continuous wave (cw) ring dye laser has also been demonstrated. High-resolution, injection-seeded OPO and optical parameter generator (OPG) systems have been developed for infrared differential absorption spectroscopy. Single-mode, tunable pulsed systems in which an external-cavity diode laser is used to injection seed a modeless dye laser are known, as well as systems that utilize four β-BBO crystals, but without a resonant cavity, injection-seeded with an 810-nm ECDL at the idler wavelength. One advantage of this system is that the operation of the system is simpler, although the use of four β-BBO crystals is required to obtain output pulse energies of several millijoules.
Thus, there is a need for a compact, affordable laser source for the generation of tunable, high-resolution pulsed radiation over a wide range of wavelengths which would be a major step forward in the field of combustion diagnostics and for other laser spectroscopic applications.